First off, it's been established that basing actual in-game speed on this (or any) demo, as it's namely a demo, is unreliable.
Secondly, we don't even know what is meant from one end of the map to the other. Is it from one corner to the other, because that increases the distance? Is it from North to South or East to West?
Furthermore, the devs are clearly talking about distance in a straight line that also includes inaccessible areas of the map. For Skellige, that includes water etc. which you wouldn't really be able to use a horse at anyway. The point is to give a basic idea of the size of the world. It's a general statement and should be taken as such. I'm not even sure if that statement was made for NML + Skellige or just one of those maps. If it's both, and we know that they're both of almost the same size, when you include water, then if it takes you 15-20 minutes in the demo to cross the entirety of Novigrad + NML, and you add Skellige to that, then that statements starts to look realistic even if movement wasn't sped up.
Finally, with all of the above said, the developers are the only ones that actually have the full informtation, they know exactly how big their map is and what horse speed at full gallop is. I'm not saying the claim of 40 minutes at full gallop is definitely correct, all I'm saying is we shouldn't make bold statements based on data that is incomplete, and without knowing exactly what was meant by that statement in the first place.